This invention relates to a method and apparatus for the cooling of a thermoplastic material tubular stalk as it is being extruded from an extruder. More particularly, the invention relates to polypropylene stalk cooling applied to a polypropylene stalk moving vertically upwards. In the production of polypropylene film from polypropylene resin, the resin is usually extruded from an extruder die in the form of a tube or cylinder of molten polypropylene. This tube or cylinder of polypropylene is caused to move along and around a cooling mandrel for solidification or crystallization of the polypropylene. After the polypropylene tube is caused to solidify, it is pulled along the mandrel over a disc tube seal through a pair of nip rolls. Above the nip rolls the tube is reheated and air is introduced in to the tube to biaxially stretch the tube into a large bubble formation. In this kind of a process, it is important that the polypropylene material be stretched at a temperature below its crystalline melting point to insure an increase in strength on stretching. Consequently, the polypropylene is first quenched or cooled by drawing it over the cooling mandrel at a rate which tends to stretch the stalk. This causes a formation of very small crystallites in the polypropylene and enables the polypropylene to be readily stretched when reheated to a stretching temperature.
Utilization of solely internal cooling of the mandrel to cool the polypropylene stalk by metal contact is limited because of the cooling capacity of the mandrel within a satisfactory axial length, instability of sliding friction between the stalk and the mandrel, and an overall non uniform cooling which causes consequent imperfections in the film made from the stalk.